Types of Rendering
This type of rendering is deemed as the most “realistic” one, as it imitates the soft plane changes of the reality. All of the necessary brush stroke transitions are blended in to create a smooth effect.
This type of rendering, unlike smooth rendering, does not worry too much about how forms exactly appear in real life. It has a much more stylistic and graphic approach, and can vary greatly. You could do the rendering through hatching, blocking in with harsh edges, or simply by using pre-made textured brushes.
Although it gives a sense of 3-dimensionality, it does not completely imitate real-life forms. It is usually created with the line art being blended in or being painted over with thin layers. This style of rendering could also be used with a combination of textured rendering.
What you could be doing wrong
1. Excessive Reliance on Soft Brushes
When you want to create a more realistic look to your drawings, especially with smooth rendering, you want to use soft transitions to imitate the organic shapes around us. However, real life consists of both harsh and soft edges. If you attempt to go over each and every shadow with an airbrush, your piece will lose clarity, and it will give an overall muddy look. Unless your purpose is to give an icy glass effect, it is the best to stay away from such a reliance on soft edges.
How to Fix it
Do not be afraid of using harsh edges! It would be easier if you started out with them first, and then added the soft shading later on. We tend to be timid with harsher and darker strokes, so you need to overcome that.
One way of showing the edges is to blend your lineart with the coloring layer underneath. However, you need to be careful as to not overblend and lose clarity from the lineart too. Also, make sure that your line art is not completely black (if you are working on a colored piece). Otherwise, your drawing will look muddy and ashy.
Try to observe the edges and plane changes on the human face. You will notice that some parts have definitive shadows. Do not be afraid of showing them.
Another great exercise is to use a fully opaque brush and not use the blender tool. It does sound scary at first but it will push you into using edges and prevent you from blending your work to a point that the clarity is lost in your piece. Furthermore, a visible structure will appear in your work that you will eventually have to force yourself to follow. You cannot show structure with completely soft strokes. Remember: you can always exclude soft edges in your art but you cannot exclude harsh edges (unless you are working on very abstract art with the intention of showing lacking of clarity) If you feel like the edges are too harsh, you can always slightly blend them in or use transitional colors to create a smoothening illusion.
2. Lack of Hue Variation
Another thing that is overlooked is hue variation. Most beginner artists use the color black to add shadows, as it is believed that objects in real life differ only in terms of value. However, this assumption fails to consider bouncing lighting and ambient occlusion, and causes paintings to look "dull" and "muddy".
Such beginner artists often stick to a single base color and do not add variety to it. Especially if they are painting skin, the subject looks dead since it lacks the natural flushes and the cool tint of veins peaking through, not to mention the light bouncing off of the surroundings (such as clothing, the sky, jewelery and etc.).
How to Fix it
Try to really observe how shadow and lighting works. You will start to see that shadows can have blues, purples, reds, greens in them- possibly all at once. Nothing stays in the same hue. Take those colors and exaggerate them, do not be timid. Add a new layer and maybe even play around with some modes. Airbrush over certain areas with different colors, and do not be afraid that they will look "irrelevant". With the correct dose and location, it will look natural and your drawing will immediately pop.
Compare the drawing on the left to the one above. Objectively, the top one looks a lot less interesting to look at, and seems "unnatural". The improved version, however, has various tints that make some areas pop- such as the flush of the cheeks and the blue tint on the chin. This improvement was done simply with colored airbrushes, and could even be improved more. Just make sure to not add the colors randomly, or they will look like random patches of colors.